I. Field
The present invention relates generally to communication, and more specifically to techniques for performing erasure detection in a wireless communication system.
II. Background
In a wireless communication system, a wireless device (e.g., a cellular phone) communicates with one or more base stations via transmissions on the downlink and uplink. The downlink (or forward link) refers to the communication link from the base stations to the wireless devices, and the uplink (or reverse link) refers to the communication link from the wireless devices to the base stations. In a Code Division Multiple Access (CDMA) system, a base station can transmit data to multiple wireless devices simultaneously. The total transmit power available at the base station determines the downlink capacity of the base station. A portion of the total transmit power is allocated to each active wireless device such that the aggregate transmit power used for all active wireless devices is less than or equal to the total transmit power.
To maximize downlink capacity, a power control mechanism is typically used for each wireless device. The power control mechanism is normally implemented with two power control loops, which are often called an “inner” loop and an “outer” loop. The inner loop adjusts the transmit power used for data transmission to the wireless device such that the received signal quality, which may be quantified by a signal-to-noise-plus-interference ratio (SIR), is maintained at an SIR target. The outer loop adjusts the SIR target to achieve a desired level of performance, which may be quantified by a block error rate (BLER) target and/or some other performance criterion.
The outer loop adjusts the SIR target based on the status of data blocks received by the wireless device. The outer loop typically decreases the SIR target by a small DOWN step if a “good” data block is received and increases the SIR target by a large UP step if a “bad” data block is received. The DOWN and UP steps are selected based on the BLER target and possibly other factors. This SIR target adjustment assumes that the status of each received data block can be reliably determined. This can normally be achieved by generating and appending a cyclic redundancy check (CRC) value to each data block prior to transmission. The wireless device can then check each received data block based on its CRC value to determine whether the data block was decoded correctly (good) or in error (bad).
A CDMA system may support data transmission using multiple transport channels and/or with multiple formats. One transport channel may carry data blocks continually and may use formats that require a CRC value to be appended to each data block sent on that transport channel. Another transport channel may be operated in a non-continuous manner so that data blocks are not transmitted some or most of the time on that transport channel. This non-continuous transmission is commonly called discontinuous transmission (DTX). No data blocks are transmitted on the transport channel during periods of no transmission, and the non-transmitted blocks are called DTX blocks. Power control for a data transmission on an intermittently active transport channel is challenging. This is because it may be difficult to accurately ascertain the true status of each received block on such a transport channel—whether the received block is a good block, a DTX block, or a bad block.
There is therefore a need in the art for techniques to reliably determine the status of each received block for an intermittently active transport channel.